add new lint: for_unbounded_range

CHANGELOG.md

@@ -6472,6 +6472,7 @@ Released 2018-09-13
 [`for_loop_over_option`]: https://rust-lang.github.io/rust-clippy/master/index.html#for_loop_over_option
 [`for_loop_over_result`]: https://rust-lang.github.io/rust-clippy/master/index.html#for_loop_over_result
 [`for_loops_over_fallibles`]: https://rust-lang.github.io/rust-clippy/master/index.html#for_loops_over_fallibles
+[`for_unbounded_range`]: https://rust-lang.github.io/rust-clippy/master/index.html#for_unbounded_range
 [`forget_copy`]: https://rust-lang.github.io/rust-clippy/master/index.html#forget_copy
 [`forget_non_drop`]: https://rust-lang.github.io/rust-clippy/master/index.html#forget_non_drop
 [`forget_ref`]: https://rust-lang.github.io/rust-clippy/master/index.html#forget_ref

clippy_lints/src/declared_lints.rs

@@ -271,6 +271,7 @@ pub static LINTS: &[&::declare_clippy_lint::LintInfo] = &[
     crate::loops::EXPLICIT_INTO_ITER_LOOP_INFO,
     crate::loops::EXPLICIT_ITER_LOOP_INFO,
     crate::loops::FOR_KV_MAP_INFO,
+    crate::loops::FOR_UNBOUNDED_RANGE_INFO,
     crate::loops::INFINITE_LOOP_INFO,
     crate::loops::ITER_NEXT_LOOP_INFO,
     crate::loops::MANUAL_FIND_INFO,

clippy_lints/src/loops/for_unbounded_range.rs

@@ -0,0 +1,33 @@
+use super::FOR_UNBOUNDED_RANGE;
+use clippy_utils::diagnostics::span_lint_hir_and_then;
+use clippy_utils::higher;
+use rustc_hir::Expr;
+use rustc_lint::LateContext;
+use rustc_span::Span;
+
+pub fn check<'tcx>(cx: &LateContext<'tcx>, arg: &'tcx Expr<'tcx>, span: Span) {
+    if let Some(range) = higher::Range::hir(cx, arg)
+        && let Some(range_start) = range.start
+        && let None = range.end
+        && let ty = cx.typeck_results().expr_ty_adjusted(range_start)
+        && (ty.is_integral() || ty.is_char())
+    {
+        let until_max = format!("={ty}::MAX");
+
+        span_lint_hir_and_then(
+            cx,
+            FOR_UNBOUNDED_RANGE,
+            arg.hir_id,
+            span,
+            "for loop on unbounded range (`0..`)",
+            |diag| {
+                diag.span_suggestion_verbose(
+                    arg.span.shrink_to_hi(),
+                    "for loops over unbounded ranges will wrap around, consider using `start..=MAX` instead",
+                    until_max,
+                    rustc_errors::Applicability::MachineApplicable,
+                );
+            },
+        );
+    }
+}

clippy_lints/src/loops/mod.rs

@@ -4,6 +4,7 @@ mod explicit_counter_loop;
 mod explicit_into_iter_loop;
 mod explicit_iter_loop;
 mod for_kv_map;
+mod for_unbounded_range;
 mod infinite_loop;
 mod iter_next_loop;
 mod manual_find;
@@ -785,6 +786,35 @@ declare_clippy_lint! {
     "using the character position yielded by `.chars().enumerate()` in a context where a byte index is expected"
 }
 
+declare_clippy_lint! {
+    /// ### What it does
+    /// Checks for unbounded for loops over char or integers.
+    ///
+    /// ### Why is this bad?
+    /// Using a unbounded range over char and integers will unexpectedly not handle overflows so it will lead to panics
+    /// or infinite loops.
+    ///
+    /// Instead there should be a max value set, usually the `MAX` constant for a given type such as `'\0'..char::MAX`
+    /// or `250..u8::MAX`.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// for i in 250u8.. {
+    ///   println!("{i}");
+    /// }
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// for i in 250u8..=u8::MAX {
+    ///   println!("{i}");
+    /// }
+    /// ```
+    #[clippy::version = "1.94.0"]
+    pub FOR_UNBOUNDED_RANGE,
+    nursery,
+    "using a for loop over unbounded range (`0..`)"
+}
+
 pub struct Loops {
     msrv: Msrv,
     enforce_iter_loop_reborrow: bool,
@@ -823,6 +853,7 @@ impl_lint_pass!(Loops => [
     INFINITE_LOOP,
     MANUAL_SLICE_FILL,
     CHAR_INDICES_AS_BYTE_INDICES,
+    FOR_UNBOUNDED_RANGE,
 ]);
 
 impl<'tcx> LateLintPass<'tcx> for Loops {
@@ -951,6 +982,7 @@ impl Loops {
         manual_find::check(cx, pat, arg, body, span, expr);
         unused_enumerate_index::check(cx, arg, pat, None, body);
         char_indices_as_byte_indices::check(cx, pat, arg, body);
+        for_unbounded_range::check(cx, arg, span);
     }
 
     fn check_for_loop_arg(&self, cx: &LateContext<'_>, _: &Pat<'_>, arg: &Expr<'_>) {

tests/ui/for_unbounded_range.fixed

@@ -0,0 +1,19 @@
+#![warn(clippy::for_unbounded_range)]
+
+fn do_something<T: Copy>(_t: T) {}
+
+fn main() {
+    for i in 0_u8..=u8::MAX {
+        do_something(i);
+    }
+
+    for i in 0_u8..=u8::MAX {
+        //~^ for_unbounded_range
+        do_something(i);
+    }
+
+    for i in '\0'..=char::MAX {
+        //~^ for_unbounded_range
+        do_something(i);
+    }
+}

tests/ui/for_unbounded_range.rs

@@ -0,0 +1,19 @@
+#![warn(clippy::for_unbounded_range)]
+
+fn do_something<T: Copy>(_t: T) {}
+
+fn main() {
+    for i in 0_u8..=u8::MAX {
+        do_something(i);
+    }
+
+    for i in 0_u8.. {
+        //~^ for_unbounded_range
+        do_something(i);
+    }
+
+    for i in '\0'.. {
+        //~^ for_unbounded_range
+        do_something(i);
+    }
+}

tests/ui/for_unbounded_range.stderr

@@ -0,0 +1,32 @@
+error: for loop on unbounded range (`0..`)
+  --> tests/ui/for_unbounded_range.rs:10:5
+   |
+LL | /     for i in 0_u8.. {
+LL | |
+LL | |         do_something(i);
+LL | |     }
+   | |_____^
+   |
+   = note: `-D clippy::for-unbounded-range` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::for_unbounded_range)]`
+help: for loops over unbounded ranges will wrap around, consider using `start..=MAX` instead
+   |
+LL |     for i in 0_u8..=u8::MAX {
+   |                    ++++++++
+
+error: for loop on unbounded range (`0..`)
+  --> tests/ui/for_unbounded_range.rs:15:5
+   |
+LL | /     for i in '\0'.. {
+LL | |
+LL | |         do_something(i);
+LL | |     }
+   | |_____^
+   |
+help: for loops over unbounded ranges will wrap around, consider using `start..=MAX` instead
+   |
+LL |     for i in '\0'..=char::MAX {
+   |                    ++++++++++
+
+error: aborting due to 2 previous errors
+